Localization of a transmitter is important in many applications. For example, in an emergency situation (e.g., a fire within a large building), having the ability to monitor the location of emergency personnel, such as police, fire-fighters etc., at an external monitoring unit may enable better coordination and/or safety.
In outdoor scenarios, transmitters may be localized by attaching GPS devices to the transmitters. However, the reception of GPS signals is unreliable in most structures and buildings, and thus, localizing transmitters inside structures and buildings using GPS devices is difficult, and in some cases, impossible.
Conventional methods for localizing transmitters within structures or buildings are classified into three categories: time of arrival (TOA), angle of arrival (AOA) and signal strength based technologies. Each method is implemented within existing networks using technologies such as, GSM, 802.11b, Bayesian based localization or ultra-wideband (UWB) techniques. However, these conventional methods generally require more than one receiver to provide accurate localization of a transmitter, or are unable to locate transmitters within structures or buildings, as is the case with GPS signals because these signals are often corrupted by delay spreading and attenuation experienced on building penetration and scattering.
Because of these limitations, other technologies have been developed to locate and track users or objects in an in-building environment. One such system uses tags placed on the items that are to be tracked. The tags can be either active or passive. An active tag contains power circuitry, which can communicate with base stations. A passive tag contains no internal power, rather it is charged either inductively or electromagnetically as it passes within the range of a base station. Using this derived power, the passive tag communicates with the station. The base stations are physically linked together through a wired or wireless network. Each tag transmits a code uniquely identifying itself. Thus, the location of the tag is determined to be in the vicinity of the base station with which the tag last communicated.
Such tag-based tracking and location systems, while being useful in an in-building environment, require a significant installation of specialized base stations. A tag-based system can only determine the location of the tags as being “near” a particular base station, consequently, to achieve a sufficiently high resolution, a large number of base stations must be installed. Obtrusive tags have to be placed on every item that is to be tracked or located, and in the case of infra-red tags, the system operates only when there is a “line-of-sight” between the tag and a base station. For all these reasons location-determination technology based on tags has had very limited success.